Mm. I don't want Luffy to achieve Gear Fourth just yet; he should learn to use his Haki alongside his Devil Fruit abilities for now, and maybe a long time in the future we'll have a Gear Fourth. Who knows. Let's just hope it doesn't become like DBZ, Bleach, and Naruto, where Luffy 'powers up' via a lot of screaming and Haki-use >_> That would ruin One Piece INSTANTLY. Part of the reason OP is good in my mind is he uses actual willpower, true EFFORT, and has to persevere and find out his opponent's weaknesses and such instead of just screaming and getting some mad boost in strength like Ichigo does and beating everyone up.